Solid-state lasers, and particularly neodymium lasers, have come to be widely used in diverse fields for numerous applications. The popularity of neodymium lasers stems largely from their versatility and simplicity. Consequently, a vast array of differing types are now commercially available.
All regenerative amplifiers generally include a laser medium which is fabricated from glass or a crystalline material such as yttrium lithium fluoride (YLF) or yttrium aluminum garnet (YAG). Included in the laser medium is a small percentage of neodymium ions which may be excited to create an electron inversion within the laser medium. Once the inversion exists, the laser medium acts as a photon amplifier, emitting a number of new photons in proportion to the number of photons absorbed. With this mechanism, regenerative amplifiers generate a stream of photons that are emitted as an amplified laser output beam.
Operation of a neodymium based regenerative amplifier requires that the neodymium ions within the laser medium be excited or caused to adopt an energy level which is greater than the ground level of the ions. This process is known as pumping, and many different techniques and methodologies exist for pumping neodymium based lasers. For instance, it is possible to pump a neodymium laser medium using broad-spectrum light sources or continuous tungsten arc lamps. In practice, however, the use of pumping devices of these types suffers from limited efficiency as a result of the inability of the neodymium ions to absorb the full spectrum of radiation emitted by the light source or lamp.
In a similar fashion, flashlamps of various types have been employed to pump neodymium lasers. Flashlamps produce large quantities of emitted radiation allowing high-power neodymium lasers to be developed. Like other broad-spectrum sources, however, the use of flashlamps generally results in low laser efficiency since much of the spectrum produced by the flashlamp cannot be absorbed by the neodymium ions.
A more efficient approach has been to pump the regenerative amplifier with a second laser source such as, for example, a gallium-arsenide semi-conductor based laser diode. Pumping with a laser diode allows the output spectrum of the pump to be matched with the frequencies most easily absorbed by the neodymium ions in the laser medium. In this way, the inefficiency inherent in the use of broad-spectrum pumping devices is avoided. Limited power outputs associated with semi-conductor laser diodes have, however, complicated development of high-power, semi-conductor pumped neodymium lasers.
Several techniques have been employed to increase the power output of semi-conductor pumped neodymium lasers. The most important technique is embodied in designs where a number of semi-conductor laser diodes are simultaneously employed to pump a single laser medium. By combining the output of a group of semi-conductor lasers, the overall power output achieved by the laser medium may be increased, and in some cases power outputs equivalent to flashlamp pumped units have been achieved.
Typically, most neodymium lasers which employ a number of semi-conductor lasers for pumping have been of the side-pumping variety. Laser designs of this type feature a number of semi-conductor laser diodes aligned alongside a rod-shaped laser medium. The beams from the diodes are aimed to pass through the side of the rod and excite the neodymium within the medium. In practice, laser designs of this type suffer from several disadvantages including high levels of noise in the generated laser output and low efficiency. Additionally, laser designs of this type are also often characterized by a phenomena known as beam pointing instability caused by variations in the beam path within the laser cavity.
To surmount the difficulties associated with side-pumped designs, end-pumped neodymium laser designed have been developed. Lasers of this type deploy a laser diode or a group of laser diodes at the end of a rod-shaped laser medium. The beams from the diodes are focused to pass through the end of the rod and excite the neodymium within the medium. Unlike side-pumped designs, however, the pumping beams on end-pumped lasers are aligned to be substantially colinear to the fundamental laser mode. Generally, end-pumped laser designs avoid the noise, inefficiency and beam pointing instability common to side-pumped lasers. In practice, however, there has been substantial difficulty involved in focusing large numbers of semi-conductor lasers to effectively end-pump a single laser rod. As a result, end-pumped laser designs often have limited power outputs. Additionally, the small focusing area produced by end-pumped designs often creates thermal inefficiency within the pumping system and degraded quality of the laser output beam due to the effects of thermal lensing.
In light of the above it is an object of the present invention to provide a system for pumping a laser medium which can be used to produce a highly efficient and relatively powerful neodymium laser. Another object of the present invention is to provide a regenerative amplifier which exhibits a relatively high power and low noise output. Another object of the present invention is to provide a system for pumping a laser medium which may be used to produce a neodymium laser producing a low noise output which is relatively free from beam pointing instability. Still another object of the present invention is to provide a system for pumping a laser medium which avoids the effects of thermal lensing in the pumped laser medium. Yet another object of the present invention is to provide a system for pumping a laser medium which is simple to operate, is relatively easy to manufacture, and which is comparatively cost effective.